The polymer network in a tough and stiff hydrogel consists of polyacrylamide (gray lines), short-chain alginate (green lines), and long-chain alginate (black lines). The polymer chains are held together by covalent bonds (blue squares) and calcium ions that form ionic bonds (red circles). The high concentration of short-chain alginate provides stiffness, while the bridging action of the long-chain alginates provides toughness. When a crack forms (notch), the ionic bonds can unzip (pink region) to dissipate stress.

Credit: ACS Macro Lett.

Chain Link

The polymer network in a tough and stiff hydrogel consists of polyacrylamide (gray lines), short-chain alginate (green lines), and long-chain alginate (black lines). The polymer chains are held together by covalent bonds (blue squares) and calcium ions that form ionic bonds (red circles). The high concentration of short-chain alginate provides stiffness, while the bridging action of the long-chain alginates provides toughness. When a crack forms (notch), the ionic bonds can unzip (pink region) to dissipate stress.

Credit: ACS Macro Lett.

Hydrogels, soft materials consisting of polymer and water, could replace damaged cartilage in people or serve as artificial muscles and nerves for soft, deformable robots. But it’s difficult to make the materials both stiff and tough at the same time; stiffer hydrogels are brittle, while those less apt to break are too squishy. Now a team at Harvard University has used a substance derived from seaweed to make a hydrogel that is both stiff and tough (ACS Macro Lett. 2014, DOI: 10.1021/mz5002355).

Joost J. Vlassak, Zhigang Suo, and their colleagues created the hydrogel by mixing distilled water and acrylamide, along with two types of naturally occurring sodium alginate, which comes from the cell walls of brown algae. They added a combination of calcium sulfate and calcium chloride to act as ionic cross-linkers in the polymer network. The two alginates had different lengths, one up to nine times as long as the other. The short-chain alginates produce a low viscosity mixture, making it easier to pack high concentrations of the polymer into the hydrogel, which increases the material’s stiffness. The long-chain versions contribute to the gel’s toughness. They form long stretches of cross-links between the other polymers, which can bridge cracks in the network. Also, these ionic links can unzip to dissipate stress.

With the right proportions of components, the new material can have an elastic modulus, a measure of stiffness, of approximately 1 megapascal, and a fracture energy, a measure of toughness, of about 4,000 J/m2. Vlassak says the material is as stiff as cartilage, but is tougher than the tissue, which has a fracture energy of 1,000 J/m2.

The properties of the hydrogel might be tuned further by using different lengths of alginates, Vlassak says. The range of lengths of naturally occurring alginates is limited, but hitting them with gamma radiation can produce different chain lengths.